The present invention relates to image display systems, and more particularly to a system for displacing characters by a diddle raster scanning technique wherein the size of the characters being displayed may be varied.
Several techniques are currently in use to display characters on the screen of a cathode ray tube (CRT). One approach utilizes conventional television raster scanning techniques to scan the entire face of the CRT, line by line, from the top to the bottom. This technique is widely used and has a number of inherent advantages. Certain applications, however, require that the size of the text being displayed on the face of the screen be of variable size. In ad layout systems and certain photocomposition systems, for example, different portions of the displayed text must be of different point sizes. This is not easily accomplished in a conventional raster scanning system, however.
A second technique, sometimes referred to as diddle raster scanning, has been used in these applications. In this technique the display is generated on the screen by scanning one full character at a time. In other words, the electron beam is moved to a particular position on the screen, and then a single character field is raster scanned at that positions so as to display an entire character. The electron beam then proceeds to display, or "paint", the next character, and then the next, and so on until all of the characters have been painted on the screen. When this technique is used, the size of the character may be changed by the simple expedient of varying the size of the minor raster which is scanned by the electron beam. This increases the size of the character field, and results in a corresponding increase in the size of the character being displayed.
The system described in Frederickson et al., U.S. Pat. No. 3,872,460, is an example of a system employing this latter technique. It was found, however, that the extent to which a character could be increased in size by this technique was limited. As the size of the character increased, the individual display strokes which made up the character moved somewhat further apart. Since the electron beam maintained a constant cross section as the size of the character was varied, this increase in the separation between the strokes of the characters resulted in the larger point size characters having a quite disjointed appearance.